Semiconductor structures are formed by sequential processing steps that process wafers where each wafer has many dies. The dies are then packaged to form the packaged semiconductor devices with each die having one or more primary functions.
The processing steps for semiconductor devices use different masks to define the different parts of each device. The mask patterns are usually on a glass or other transparent substrate so that optical, X-ray or other energy sources are imaged through the mask onto a prepared wafer surface. A set of different masks are used in sequential steps of the semiconductor manufacturing processes. The processing of the wafer and each of the dies on a wafer results in dies that have one or more primary functions according to an engineering design. The processing steps performed to create the dies are referred to as the “native” processing steps.
A semiconductor device may have a radio frequency (RF) function as its primary function. In one example, the RF function for a tag for tracking information is the primary function of the semiconductor device that forms the structure of the tag.
A RF communication for an RF function is implemented with an RF element, having an RF transmitter and an RF receiver, as a part of the semiconductor device. For a tag semiconductor device, the RF element typically communicates with an external RF communicator such as a tag reader.
Regardless of the particular function of the RF element, the RF communication requires an antenna. Antennas that are an integral part of a semiconductor device are defined as internal antennas. RF communication with an internal antenna of a semiconductor device is done by having the internal antenna communicating with an antenna of an RF communicator (sometimes called interrogator, reader, or writer) or with an antenna of some other device.
The antenna is a key part of RF communication. In typical RFID (radio frequency identification) systems, an antenna of a RF tag is an element that permits the tag to exchange data with the antenna of an RF communicator. The exchange can occur through inductive magnetic field coupling between the antennas. Many tags in RFID systems operate as passive devices wherein the tag is energized upon the coupling of the tag antenna with the communicator antenna. The response of a tag occurs by changing the load of the magnetic field in the RF tag. For example, some passive RFID tags make use of a coiled tag antenna that creates a magnetic field using the energy provided by the external RF communicator's carrier signal.
For an RFID tag formed as a discrete semiconductor device where RF communication is the primary function of the semiconductor device, the area of tag is very small (typically with a die area in the range of from about 0.5 mm2 to about 0.10 mm2). Antennas internal to such tags (“built-in antennas”) that use the native manufacturing processes of the semiconductor device have been small antennas limited in size to the size of the semiconductor device. Such small antennas, however, do not perform adequately because only a short communication range to the reader is possible or because of other RF communication problems inherent in very small antennas.
To achieve a greater range of interaction with an external RF communicator and achieve better antenna performance, RFID tags typically have larger “off-tag” antennas. The off-tag antennas require special manufacturing steps that are different than the native semiconductor processing steps used in manufacturing the tag. Therefore, “off-tag” antennas increase the overall RFID manufacturing cost. While it is desirable to build internal antennas for semiconductor devices using native processing steps, the antennas ultimately must be capable of RF communications that satisfy the RF communication function in a low cost and efficient manner.
The size, structure and designs of antennas are critical to achieving reliable, low cost RF communication functions in semiconductor devices.
In communication devices generally and specifically for semiconductor devices, antennas are elements having the function of transferring radiation energy to (in the receive mode) or from (in the transmit mode) an electronic device. Radiation energy is transferred from an electronic device into space or is transferred from space into the electronic device. A transmitting antenna is a structure that forms a transition between guided waves constrained by the electronic device and space waves traveling in space external to the electronic device. A receiving antenna is a structure that forms a transition between space waves traveling external to the electronic device and guided waves constrained by the electronic device. Often the same antenna operates both to receive and transmit radiation energy.
Frequencies at which antennas radiate are resonant frequencies. A resonant frequency, f, of an antenna can have many different values as a function, for example, of components providing the signal to the antenna, conductors of the antenna, dielectric constants and other electrical properties of materials of the antenna, the type of antenna, the size and other geometry of the antenna and the speed of light.
In general, wavelength, λ, is given by λ=c/f=cT where c=velocity of light (3×108 meters/sec), f=frequency (cycles/sec), T=1/f=period (sec). The radiation wavelength, λ, of an antenna is related to physical dimensions of the antenna. The electrical impedance of an antenna is allocated between a radiation resistance, Rr, and an ohmic resistance, Ro. The higher the ratio of the radiation resistance, Rr, to the ohmic resistance, Ro, the greater the radiation efficiency of the antenna.
A number of different small antenna types are well known and include, for example, loop antennas, small loop antennas, dipole antennas, folded dipole antennas, stub antennas, conical antennas, helical antennas and spiral antennas. Small antennas, including loop antennas, often have the property that radiation resistance, Rr, of the antenna decreases sharply when the antenna length is shortened. Small loops and short dipoles typically exhibit radiation patterns of λ/2 and λ/4, respectively. Ohmic losses due to the ohmic resistance, Ro are typically minimized using impedance matching networks. Although impedance matched small loop antennas can exhibit 50% to 85% efficiencies, their bandwidths have been narrow, with very high Q, for example, Q>50. Q is defined for purposes of this specification as follows:Q={transmitted or received frequency}/{3 dB bandwidth}.
An antenna goes into resonance where the impedance of the antenna is purely resistive and the reactive component is 0. Impedance is a complex number consisting of real resistance and imaginary reactance components. A matching network can be used to force resonance by eliminating the reactive component of impedance for particular frequencies.
By way of one example, a short dipole antenna has an equivalent electrical circuit that is a series RLC connection where the inductance L represents the inductance of the antenna conductors, the capacitor C is the capacitance between the antenna conductors and the resistance R represents the energy of radiation. When the dipole is very short, the circuit is dominated by the capacitive reactance and the radiation resistance with the radiation resistance being very small and the capacitive reactance quite large. As the antenna is made longer, the radiation resistance increases as well as the inductive reactance while the capacitive reactance becomes smaller. When the antenna is approximately one half wavelength long, the capacitive and inductive reactances are equal, they cancel, and the antenna is in resonance. As an antenna becomes longer still, the equivalent circuit transforms into a parallel resonate circuit of R, L and C values. The resonance point is reached when the antenna is about one wavelength long and the radiation resistance becomes very high. When the antenna approaches one and one half wavelengths it again looks like a series RLC circuit and at two wavelengths it is back to the parallel circuit, This impedance pattern repeats in increments of one wavelength in length.
Internal antenna designs have included Microstrip Antennas, Patch Antennas, Planar Inverted-F Antennas (PIFA) and Meander Line Antennas (MLA).
Microstrip Antennas are similar to monopole antennas, except Microstrip Antennas are positioned on a two-dimensional surface such as a circuit board internal to a wireless device. Usually Microstrip Antennas are designed based upon ½-wavelength, λ/2, or ¼-wavelength, λ/4, conductor geometry. Although such antennas are inexpensive, they suffer in radiation inefficiency due to surrounding metallic and other sections. Microstrip Antennas are usually limited in bandwidth to narrow, single-frequency band applications.
Patch Antennas are typically fabricated out of a square or round conductive plate mounted parallel to and offset over a ground plane. In a typical example, the dimension of the square plate is precisely ½-wavelength, λ/2. The resulting radiation pattern is normal to the surface of the ground plane (typically the circuit board of a device), resulting in a directional “mushroom” or “inverted cone” pattern that tends to be narrow in bandwidth. Such antennas are typically used in single frequency applications requiring a directed beam pattern, such as a wall-mounted or reader access point.
PIFA Antennas are in the shape of a letter “F” lying on its side with the top mounted parallel to and offset over a ground plane, with the two shorter sections mounted perpendicular to the top to provide feed and ground points and with the ‘tail’ providing the radiating surface. PIFA antennas exhibit omni directional patterns and can radiate in more than one frequency band, but their efficiencies are not good and they are difficult to design.
Meander Line Antennas (MLA) are a combination of a loop antenna and a frequency-tuning meander line. The electrical length of the MLA is made up principally by the delay characteristic of the meander line rather than the length of the radiating structure itself. MLAs can exhibit broadband capabilities for operation on several frequency bands.
While many different types of antennas are known, the embedding of internal antennas in semiconductor structures using native processing provides special challenges in order to achieve satisfactory radiation and other antenna performance without interfering with the semiconductor structures utilized for the primary function.
The area of discrete RF element (e.g. a tag) is typically very small (typically of an area in the range of about 0.1 mm2 to about 0.50 mm2). An antenna internal to such a tag or device supporting RF communication therefore has been limited to sizes that are a small fraction of the desired radiation frequency wavelength, λ. For example, semiconductor devices with areas of from about 0.1 mm2 to about 0.5 mm2 have sides of from about 0.32 mm to about 0.71 mm, respectively, so that peripheral loop antennas (with four sides used) have lengths from about 1.2 mm to about 2.8 mm, respectively. For a communication frequency of about 900 MHz with a wavelength, λ, of about 30 cm, the peripheral loop antennas having lengths from about 1.2 mm to about 2.8 mm are related to wavelength, λ, with ratios from about λ/250 to λ/110, respectively. Normally, for RF communication with small antennas having simple geometries, wavelengths of λ/32, λ/16, λ/4, λ2 and λ, are preferable. The wavelengths having ratios from about λ/250 to about λ/110 are far from the preferable values and hence generally are not expected to provide good antenna performance. More particularly, none of the antenna architectures that are used in embedded antenna designs for semiconductor devices have proved entirely satisfactory for reasons such as limited bandwidth, poor efficiency, high cost or design difficulty.
It is desirable that RF communication elements be fabricated using the same native processing steps as conventionally used for semiconductor devices while still providing antenna structures and designs that provide good RF communications.
In light of the foregoing, there is a need for improved semiconductor structures that perform RF communication functions where the RF elements for the RF communication functions can be fabricated using the native processing steps of semiconductor devices.